The present invention relates to nuclear medical diagnosis apparatuses, and in particular relates to a positron emission computed tomography apparatus (hereinafter, referred to as a PET apparatus) that is one type of the nuclear medical diagnosis apparatuses using radiation detectors, a single photon emission computed tomography apparatus (hereinafter, referred to as a SPECT apparatus), and a γ-camera using γ-rays passing through a test object.
Conventionally, as the radiation detectors for detecting radiation, such as γ-rays, a radiation detector using an NaI scintillator is known. In the γ-camera equipped with an NaI scintillator, radiation (γ-ray) is incident upon the NaI scintillator at an angle limited by a large number of collimators, thus interacting with an NaI crystal to emit scintillation light. This light reaches a photomultiplier or a photodiode via a light guide to become an electrical signal. The electrical signal is shaped by a measuring circuit mounted on a circuit substrate and is then sent to an external data acquisition system from an output connector. In addition, these NaI scintillator, light guide, photomultiplier, measuring circuit, circuit substrate, and the like are entirely housed in a light shielding case to block electromagnetic waves other than external γ-rays.
In addition, here, when an NaI scintillator is combined with a photomultiplier or a photodiode, this combination is defined as a radiation detector.
The radiation detector that detects radiation based on a principle different from the principle of the radiation detector combining such NaI scintillator with a photomultiplier or a photodiode is a semiconductor radiation detector equipped with semiconductor radiation detection elements using a semiconductor material, such as CdTe (cadmium telluride), CdZnTe (zinc telluride cadmium), HgI2 (mercury iodide), TlBr (thallium bromide), or GaAs (gallium arsenide). In this semiconductor radiation detector, the semiconductor radiation detection element convert a charge resulting from an interaction between a γ-ray and the semiconductor material into an electrical signal, so this semiconductor radiation detector can accomplish conversions into an electric signal more efficiently than the scintillator can, and also accomplish miniaturization. Accordingly, this semiconductor radiation detector attracts much attention.
In addition, in the nuclear medical diagnosis apparatus that generates an image using a large number of such radiation detectors, there is a problem that when a noise signal mixing with an intended radiation detection signal outputted from the radiation detector is outputted from an abnormal radiation detector, i.e., a photomultiplier, a photodiode, or a semiconductor radiation detector, this noise signal is also processed during image generation.
For this reason, the conventional technique described in JP-A-2006-98411 (paragraphs [0032], and [0035] to [0039]) uses the method for detecting an abnormal semiconductor radiation detector and excluding an output signal therefrom, wherein in a PET apparatus or SPECT apparatus equipped with a plurality of semiconductor radiation detectors, semiconductor radiation detectors are arranged so as to surround the circumference of the body axis of a test object and also to be in multiple layers in the radial direction. Here, with respect to γ-rays in the radial direction passing through the semiconductor radiation detectors in multiple layers, a ratio between signals outputted by the semiconductor radiation detectors in each layer is used to determine which semiconductor radiation detector on which layer is abnormal when the ratio deviates from a predetermined ratio by a specified amount or more.
In addition, in the technique described in the above-described JP-A-2006-98411, an abnormal radiation detector can be determined only after carrying out transmission imaging or actual SPECT imaging, or PET imaging, or imaging of transmission image, and even if a noise signal is outputted from an abnormal radiation detector, a signal processing device will process an output signal including this noise signal as a radiation detection signal, thus resulting in an increase in the signal processing load of the signal processing device. Moreover, when multiple layers of semiconductor radiation detectors are disposed in the radial direction, multiple layers of semiconductor radiation detectors in the radial direction at a specific position can be determined as abnormal, but this technique can not be applied to the case where radiation detectors using a scintillator are arranged in one layer in the radial direction.
As a result, there is a problem that the generation of a SPECT image, PET image, and transmission image can not be started until the determination of a abnormal radiation detector is completed in data processing after the imaging.